Monodisperse high polymers are increasingly desirable in industries requiring stringent material characterization and quality control. Polydispersity indices (defined as the weight-averaged molar mass of a polymer sample divided by the number-averaged molar mass of the same polymer sample) of less than 1.20 are not currently achievable by controlled polymerization methods such as anionic living polymerization in polymers of molar masses that exceed about 500,000 g/mol (generally defined as “high polymers”). The definition of “narrow” molar mass distribution, “low polydispersity” or “monodisperse” for a polymer may vary with the application; however, current methods generally enable the preparation of polymer samples with polydispersity indices of approximately 1.10–1.30. Achieving polydispersities lower than 1.15 for high polymers is generally extremely difficult. A measurable polydispersity index lower than about 1.09 for a polymer with a molar mass of greater than 500,000 g/mol (i.e., for a high polymer) is difficult to achieve, particularly so with water soluble polymers.
Methodologies of the prior art to produce monodisperse polymer samples are generally based on living polymerization, a process by which prematurely terminated polymer molecules for high molar mass polymers are catalytically “revived” by inorganic ligands and carried out to a chain length near a desired value through the amount of monomer supplied. [Inoue S: Immortal Polymerization: The Outset, Development, and Application. Journal of Polymer Science-A 2000: 38: 2861–2871.] Anionic polymerization is a type of living polymerization involving the use of anionic inorganic materials as catalysts for controlled polymerization [Lee J, Hogen-Esch T E: Synthesis and Characterization of Narrow Molecular Weight Distribution AB and ABA Poly(vinylpyridine)-Poly(dimethylsiloxane) Block Copolymers via Anionic Polymerization. Macromolecules 2001: 34: 2805–2811; Hong K, Mays J W: 1,3-Cyclohexadiene Polymers. 1. Anionic Polymerization. Macromolecules 2001: 34: 782–786]. Living polymerization necessitates minimization of any side reactions, i.e., all reactions except chain growth from a living end. There is, accordingly, a strict limitation in the polymers made by this method to those without unduly reactive side chains or substituents. As a result, many classes of water-soluble polymer with reactive side chains cannot be made by living polymerization [Zhou X Z, Shea K J: Synthesis of Poly(methylene-b-styrene) by Sequential Living Polymerization. Macromolecules 2001: 34: 3111–3114; Gottfried A C, Brookhart M: Living Polymerization of Ethylene Using Pd(II) α-Dimine Catalysts. Macromolecules: 34: 1140–1142; Trzaska S T, Lee L W, Register R A: Synthesis of Narrow-Distribution “Perfect” Polyethylene and Its Block Copolymers by Polymerization of Cyclopentene. Macromolecules 2000: 33: 9215–9221; Xu G, Chung T C: Synthesis of Syndiotactic Polystyrene Derivatives Containing Amino Groups. Macromolecules 2000: 33: 5803–5809].
Difficulties are also encountered for polymers of lesser molar mass. Many chemical methods necessary to ensure monodispersity are both relatively expensive and hazardous when compared to more conventional methods of polymerization: such as common free-radical polymerization, for example, because of the metallic catalysts used [Nonaka H, Ouchi M, Kamigaito M, Sawamoto M: MALDI-TOF-MS Analysis of Ruthenium(II)-Mediated Living Radical Polymerizations of Methyl Methacrylate, Methyl Acrylate, and Styrene. Macromolecules 2000: 34: 2083–2088; Gottfried A C, Brookhart M: Living Polymerization of Ethylene Using Pd(II) α-Dimine Catalysts. Macromolecules: 34: 1140–1142; Kotani Y, Kamigaito M, Sawamoto M: Living Radical Polymerization of Para-Substituted Styrenes and Synthesis of Styrene-Based Copolymers with Rhenium and Iron Complex Catalysts. Macromolecules 2000: 33: 6746–6751; Kotani Y, Kamigaito M, Sawamoto M: Living Radical Polymerization of Styrene by Half-Metallocene Iron Carbonyl Complexes. Macromolecules 2000: 33: 3543–3549; Kubo H, Hayano S, Masuda T: Polymerization of Aliphatic Disubstituted Acetylenes by MoOCl4-n-Bu4Sn-EtOH Catalyst: Formation of Polymers with N arrow MWDs and Confirmation of the Living Character. Journal of Polymer Science-Part A: Polymer Chemistry 2000: 38: 2697–2701].
As evident from the preceding, the preparation of monodisperse high polymers has been an ongoing concern in the art. Macromolecular materials of molar masses in the range of about 105–about 107 g/mol and of a relatively narrow molar mass distribution (<1.10) would be highly desirable for a number of applications in the biomedical and pharmaceutical industries. Also, a simple, low-cost route to polymers with a narrow molar mass distribution would facilitate the manufacture and processing of materials through enhanced quality control and more narrowly defined and foreknown material properties. New applications for monodisperse polymers would likely be developed if such materials were more readily attainable.